tropisms

Tropism is the directional growth response of a plant or its organs toward or away from an external stimulus. The response is oriented with respect to the stimulus source, unlike nastic movements, which are non-directional and occur regardless of the stimulus direction. Tropisms help a plant optimize light capture, water access, and other resources and are typically controlled by plant hormones, especially auxins, which regulate differential cell elongation.

The main tropisms are:

- Phototropism: growth toward or away from light. Shoots are usually positively phototropic (toward light), while roots are typically negatively phototropic (away from light).

- Gravitropism (geotropism): growth in response to gravity. Roots show positive gravitropism (grow downward), while shoots exhibit negative gravitropism (grow upward).

- Thigmotropism: growth in response to touch or physical contact, common in climbing plants and tendrils.

- Hydrotropism: growth toward higher moisture, aiding water acquisition.

- Chemotropism: growth toward chemical signals; in flowering plants, pollen tubes show chemotropic growth toward ovules, and roots may respond to nutrient cues.

- Thermotropism: growth in response to temperature gradients; observed in some plant tissues and fungi.

Mechanistically, tropisms arise from asymmetric distribution of growth regulators, primarily auxin. For example, in phototropism, light causes a lateral redistribution of auxin to the shaded side, promoting cell elongation and bending toward the light. In gravitropism, gravity influences the movement of statoliths within the root cap or shoot tip, guiding differential auxin distribution and bending accordingly.

Tropisms influence plant form and habitat use, contributing to architecture, stability, and resource acquisition. Early foundational work on phototropism by Darwin and later researchers helped establish the role of growth hormones in directional plant responses.